Mount structure for filamentary cathodes



Feb. 25, 1964 J. M.BENNETT MOUNT STRUCTURE FOR FILAMENTARY CATHODES 2 Sheets-Sheet 1 A l I r l. ,,I UUPH Flled Jan 3l 1962 l 5 y :75l

A T TORNEY Feb. 25, 1964 `-M, BENETT :122,671

MOUNT STRUCTURE FOR FILAMENTARY CATHODES Filed Jan. 31, 1962 2 Sheets-Sheet 2 88 INVENTOR,

JOHN M. BENNETT United States Patent 3,122,671 MOUNT STRUCTURE FOR FILAMENTARY CA'IHDES John M. Bennett, New Port Richey, Fla., assigner to the United States of America as represented by the Secretary of the Army Filed Jan. 31, 1962, Ser. No. 17 0,295 1 Claim. (Cl. 313-269) This invention relates to an improved means for the suspension of lamentary cathodes in thermionic devices, such as electron tubes. It is more particularly directed to the protection from external mechanical excitation or vibration, and the prolonging of the useful life of lamentary cathodes.

In electron tubes having therrnionic cathodes made of iine metal wire that may or may not be coated with an electron emissive substance, the failure of many of these tubes can be attributed to the suspension means holding the cathodes therein. The conventional suspension means generally fail to protect the delicately suspended and vibration-prone cathodes from externally caused mechanical excitation.

The vibration of a frlamentary cathode in an electron tube that contains a control grid and an anode, for example, may produce many effects that are deleterious to the operation of the tube. These vibrations can result in rapid, more or less sustained changes in the anode current and the control grid-anode transconductance just as surely as variances in the control grid voltage can effect such changes. At audio frequencies, for instance, the vibrating cathode is easily recognized by the characteristic buzz, howl or whistle it generates. An electron tube having this defect is usually labelled a microphonic and, as such, it is not t for many circuit applications, particularly where any sort of mechanical shock must be absorbed by the electron tube in the course of its operation.

In general, it is diicult to prevent the resonant Vibration of a tine element, like a ilamentary cathode, that is stretched between two or more points. Various damper bars have in the past been mounted to bear against selected portions of such cathodes and many different tension means have been used to stretch and hold fllamentary cathodes. For example, one very common way to suspend a single strand iilamentary cathode is by having it attached at one end to a solid support and at the other end to a spring. The spring may take any of various forms, one of the most widely used springs being a straight wire fastened at one end to a solid support and at the other end to the cathode. In the suspension of lamentary cathodes that are folded into V or M shapes, for instance, the ends of the cathodes are usually fastened to solid supports and the bends are draped over springs. Besides maintaining tension on the cathodes, such devices and others merely permit some choice of frequencies at which the cathodes more readily vibrate. At best, and by careful design, some compromise resonance or vibration frequency susceptibility that is not too objectionable can be built into the lamentary cathodes by such suspension means; but the vibration problem still remains.

Many electron tube failures due to iilamentary cathode burn-outs may also be traced to the cathode suspension means. Since the ordinary lilamentary cathode is held by relatively massive metal suspension means, it follows that, in operation, a large amount of heat is conducted away from such a cathode by the suspension means.l

Since the portions of the cathode nearest the support means are so rapidly cooled, a so-called hot spot condition is usually developed on the cathode somewhere between the support means. This hot spot is operated "ice at abnormally high temperatures because the greater part of the electron emission drawn from the cathode comes from the hot spot and the immediate surrounding area. It is evident that a severe operational hot spot condition existing on a lilamentary cathode will shorten the time until the cathode will burn out or disintegrate.

Accordingly, one object of the invention is to provide an improved shock damping means of suspension for lamentary cathodes used in electron tubes.

Another object is to provide an electron tube in which the mount structure and the lamentary cathode or cathodes are held by shock damping means.

Another object is to provide apparatus to assemble such shock absorbing suspension means to lamentary cathodes in electron tube mount structures.

A further object is to provide suspension means for lilamentary cathodes in electron tubes whereby a minimum of operational heat is conducted away from the cathodes, and the hot spot temperature on the cathodes is downgraded.

For a more detailed description of the invention together with other and further objects thereof, reference is had to the following description, taken in connection with the accompanying drawing, in which:

FIG. l is a perspective view of an electron tube, certain parts being broken away to show a mount structure embodying a lamentary cathode assembly, suspension means therefor and resilient mount snubbing means, as taught in the present invention;

FIG. 2 is a detail of one of the lamentary cathode assemblies of FIG. 1 and the support means therefor;

FIG. 3 is an enlarged detail of a connector attached to an end portion of a iilamentary cathode as in FIG. 1;

FIG. 4 is a top view of a mounting jig which is used to assemble filamentary cathodes to the electron tube iount structure of FIG. l; and

FIG. 5 is a front elevation of the mounting jig and the electron tube mount structure shown in FIG. 4.

Referring now to the drawings, the electron tube illustrated in FIG. 1, as example only, is a beam power amplifier comprising a mount structure lll, a stem assembly 12 and an envelope 14. Mount structure 10 includes a control grid 16, a screen grid 18, beam-forming plates 20, and anode 22, a top shield 24, a bottom shield 26, a pair of flamentary cathode assemblies 28 and a novel suspension means therefor, in combination with a mount structure snubbing means which are within the contemplation of the present invention. The foregoing mount structure elements are held in xed spatial relationship by insulating spacers 30 and 32, which are generally made of mica. Two overlaying spacers 30 are used at the top end of the mount structure 10 and one spacer 32 is used at the bottom and nearest stem assembly 12.

Each cathode assembly 28 is tautly suspended between a pair of cantilever springs 34 and 36. Each spring 34 and 36 comprises a helical portion, a straight tangential extension therefrom at one end and a shorter radial extension at the other end. When the springs hold each cathode assembly 28 as in FIGS. 1 and 2, the angle between the extensions of each spring 34 fand 36 is approximately Springs 34 and 36 are of equal tensility and hold the cathode assemblies 28 in balanced suspension. The radial extensions from the helixes of springs 34 are fastened to surface 3S which is a folded over portion of top shield 24. The radial extensions of springs 36 are each fastened to a wire brace 40, immediately below spacer 32. A ribbon lead 42 is fastened to each brace 4G' and is used to make electrical connection to an appropriate stem lead 44. Cathode assemblies 28 are series connected and are provided with a center top connection to a stem lead 44.

s This center top connection is made through top shield 24, beam-forming plates 2t) and the bottom shield 26.

As better shown in FIGS. 2 and 3, each cathode assembly 28 includes a length of thin Wire 46 which may be of nickel alloy coated with an electron emissive coating of alkaline earths 48, and a pair of heat-retlective connectors 5t), made of bright metal, fastened respectively to the ends of wire 46. A part of each connector 50 is wrapped around an end portion of coating 48 and the wire 46 is welded to the shank of the connector. These con nectors constitute heat reflective jackets that tend to elevate the usually low operational temperatures at the end portions of the tilamentary cathodes and to cause a more uniform temperature gradient to exist along the cathodes. As the temperature and the consequent electron emission along the lamentary cathodes are made more ideally uniform, the undesirable hot spot condition is tempered to a safer level.

Each cathode assembly 23 is fastened to and is suspended between the tangential extensions from the helixes of springs -34 and 36 through its connectors 50. The cathode assemblies 28 are held solely by these means, and are not drawn tightly against the edges of the openings in mica spacers 30 and 32, either when not heated or when slightly displaced by operational heat as indicated by the dotted lines in FIG. 2.

Two oppositely-disposed snubbers 52 are laced downwardly through openings near the periphery of spacers 30, as shown. A portion of each spring snubber 52 eX- tends below and outwardly from the periphery of spacers 30 and bears against the inner surface of envelope 14. Snubbers 52 keep the upper portion of mount structure centrally disposed in envelope 14 and function as mechanical shock cushions between the envelope 14 and the mount structure 10. A number of evenly-shaped, integral, resilient points 54 project outwardly from the periphery of spacer 32. Points 54 bear against the inner surface of envelope 14 and also serve as mechanical excursion limiters.

Connections from control grid 16, screen grid 18 and anode 22 are made to the proper stem leads 44. Stem leads 44 are extensions of outer pins 56 which are used to connect the various internal elements of the electron tube of FIG. 1 to suitable external circuitry. A getter assembly 58 is mounted to lan anode projection 60' above top spacers 30 as shown. The chamber 62 formed by the sealing of stem assembly 12 to envelope 14 is evacuated through a tube 64 leading out from the top of envelope 14. After evacuation the chamber 62 is sealed by the closure of tube 64 which completes the assembly of the electron tube.

It is obvious that many different kinds of springs may be used in place of springs 34 and 36 Without departing from the concepts of the present invention. -For instance, flat leaf springs, straight wire springs, coil springs and various cantilever springs could be used as well. Different forms for the connectors 50 will also readily suggest themselves to those familiar in the art. For example, these connectors might well be made from lengths of nickel tubing of sufficient inside diameter to accommodate the coated cathode wire. In this case, electrical connection to the cathode wire could be made simply by squeezing the connectors around the bare cathode wire.

In order to practice the invention and produce electron tubes embodying the tilamentary cathode suspension means taught here with any degree of speed and accuracy compatible with present-day production standards, a specially designed mounting jig as shown in FIGS. 4 and 5 is utilized.

The mounting jig 66 of FIGS. 4 and 5 includes a base 63 on which are mounted two latches 7G, a trough 72 and two opposing jaws 74. The trough 72 is centered on base 68 and each of the latches 70' are located on opposite ends thereof. Between each latch 70 and trough 72, a

groove 8) is cut into the top surface of base 68 to hold pivotally a jaw 74. Each jaw 74 includes an upright section 82 and a head 84 at right angles thereto. A guide rod 78 extends from each latch 70 to trough 72 and is straddled by section 32. The top surface of trough 72 is provided with a longitudinal depression 94 to hold mount structure 10 in a predetermined position as shown. The edge 83 of head S4 is slotted to position and hold the respective free ends of springs 34 and 36, whose other ends are secured to mount structure 10.

Two pins 86 are attached to each upright section 82 and project from the sides thereof. Each pin 86 has a cooperating pin 88 which is fastened to the edge of base 68 and projects therefrom. Pins 83 are located inwardly of pins S6 so that tension springs 90, strung between each cooperating pair of pins 86 'and 88, force the opposing jaws 74 against mount structure 10. The top ends of latches 70 taper down to transverse grooves 76. A yoke 92 is xed to each jaw 74 at an angle, permitting the yokes to engage grooves 76 when jaws 74 are retracted, as indicated by the dotted lines in FIG. 5.

In the assembly of springs 34 and 36 to cathode assemblies 28, the mount structure 10, to which the springs 34 and 36 are already attached at one end, and which contains the unconnected cathode assemblies 28, is first positioned on its side in the depression 94 of trough 72. At this point, the jaws 74 are held retracted by latches 70, and the free end portions of springs 34 and 36 are outwardly sprung. When the jaws 74 are released from latches 7), they are drawn by the tension springs 90 against mount structure 10, so that the edges 83 of heads S4 come to rest against the outer surfaces of spacers 30 and 32, respectively. In this position, the slotted head 84- engages the respective free ends of springs 34 and 36, and bends them to a position substantially parallel to the outer surfaces of spacers 30 and 32. The respective free ends of springs 34 and 36 are now in contact with the respective extremities of connectors 50 of cathode assemblies 28. Thus, an equilibrium and a correct positioning of springs 34 and 36 are established, and the respective ends of the springs contacting the extremities of connectors 5) can be welded together. After these welds are made, jaws 74 are retracted, leaving springs 34 and 36 attached to and supporting the cathode assemblies 28. Mount structure 1) is removed from jig 66 and is further processed into a finished electron tube.

While there has been described what is at present considered a preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is therefore aimed in the appended claim to cover all such changes and modications as fall within the spirit and scope of the i11- vention.

What is claimed is:

In an electron tube having a mount structure, the cornbination of a iilamentary cathode connected at each end to said mount structure, first shock absorbing means for resiliently holding said mount structure in said tube comprising two spaced insulating spacers surrounding said mount structure and holding same in fixed spatial relationship within said tube, two oppositely-disposed spring snubbers extending outwardly from the periphery of one of said spacers and bearing against the inner surface of said tube, a plurality of evenly-spaced integral, resilient points projecting outwardly from the periphery of said other spacer and bearing against the inner surface of said tube, second shock absorbing suspension means for resiliently holding said lilamentary cathode in said mount structure comprising cantilever springs of equal tensility mounted between the ends of said tilamentary cathode and said mount structure, said filamentary cathode comprising a thin wire coated with an electron emission coating having one end of a bright metal heat-reflecting connector wrapped around a coated portion at each terminus of said wire, and said cantilever springs connected between the other end of each of said connectors and said mount structure.

References Cited in the le of this patent UNITED STATES PATENTS 5 1,689,821 Freeman Oct. 3G, 1928 2,044,369 Samuel June 16, 1936 2,055,897 Hirmann Sept. 29, 1936 2,116,224 Stermberg May 3, 1938 10 6 Miller Oct. 21, 1941 Krim Aug. 8, 1944 Wood June 25, 1946 Derby et a1. July 27, 1948 Ewing Nov. 1, 1949 Kohl June 3, 1952 Kelpp Mar. 31, 1959 Kuer Oct. 13, 1959 Forte June 2S, 1960 

